Transparent beta-quartz glass-ceramics with low lithium content

ABSTRACT

The present application provides transparent glass-ceramics of β-quartz of composition containing a small content of lithium, articles constituted at least in part of said glass-ceramics, glasses precursors of said glass-ceramics, and also a method of preparing said articles. Said glass-ceramics have a composition, free of arsenic oxide and antimony oxide, except for inevitable traces, expressed as percentages by weight of oxides, containing: 62% to 68% of SiO2; 17% to 21% of AI2O3; 1% to &lt;2% of Li20; 1% to 4% of MgO; 1% to 6% of ZnO; 0 to 4% of BaO; 0 to 4% of SrO; 0 to 1% of CaO; 1% to 5% of TiO2; 0 to 2% of ZrO2; 0 to 1% of Na2O; 0 to 1% of K2O; with Na2O+K2O+BaO+SrO+CaO &lt;6%; optionally up to 2% of at least one fining agent comprising SnO2; and optionally up to 2% of at least one coloring agent.

This application claims the benefit of priority under 35 U.S.C. § 119 of French Patent Application Serial No. 17 55049 filed on Jun. 7, 2017 the content of which is relied upon and incorporated herein by reference in its entirety.

The context of the present application is that of transparent low expansion glass-ceramics containing a solid solution of β-quartz as the main crystalline phase. The present application relates more particularly to:

-   -   transparent glass-ceramics containing a solid solution of         β-quartz as the main crystalline phase and a composition with a         low lithium content;     -   articles constituted at least in part of these glass-ceramics;     -   aluminosilicate glasses, precursors of these glass-ceramics; and     -   a method of preparing these articles.

Transparent glass-ceramics of the lithium aluminosilicate (LAS) type containing a solid solution of β-quartz as the main crystalline phase have been in existence for more than 20 years. They are described in numerous patent documents and in particular in U.S. Pat. No. 5,070,045 and patent application WO 2012/156444. They are used in particular appliances as the material for constituting cooktops, cooking utensils, microwave oven plates, fireplace windows, fireplace inserts, stove windows, oven doors (in particular for pyrolytic and catalytic oven), and fire-windows.

In order to obtain such glass-ceramics (and more precisely in order to eliminate inclusions of gas within the precursor molten glass), conventional fining agents, As₂O₃ and/or Sb₂O₃, have been in use for a long time. Given the toxicity of these two elements and the ever more severe regulations that are in force, it is desirable to avoid using these (toxic) fining agents in the fabrication of the precursor glass. SnO₂ has been proposed as a substitute fining agent (see in particular the teaching of patent documents U.S. Pat. Nos. 6,846,760, 8,053,381, and WO 2012/156444). It is being used more and more. Nevertheless, at a similar fining temperature, it is found to be less effective than As₂O₃. In general manner, and particularly in the context of using SnO₂ as a fining agent, in order to facilitate fining, it is advantageous to have (precursor) glasses that have low viscosities at high temperature.

Depending on the heating means that are associated with cooktops used (radiant heating means or induction heating means), requirements concerning values for the (linear) coefficient of thermal expansion (CTE) of the material constituting said cooktops are more or less constraining. Cooktops used with radiant heaters may be raised to temperatures as high as 725° C., and in order to withstand the thermal shocks and the thermal gradients that can arise in the cooktop, they have a CTE that generally lies in the range −10·10⁻⁷ per Kelvin (K⁻¹) to +10·10⁻⁷K⁻¹. Cooktops used with induction heaters are subjected to lower temperatures (at most about 400° C.). A new generation of inductors, using infrared sensors, has also recently appeared. Those sensors make it possible for the temperature of the cooktops to be better controlled and not to exceed 300° C. Cooktops used with induction heaters are therefore subjected to thermal shocks that are less violent; the CTE of said cooktops can therefore be greater.

For reasons of appearance, it is also desirable for a cooktop, even when transparent, to mask the elements that are placed beneath it, such as induction coils, electric wiring, and circuits for controlling and monitoring the cooking appliance. An opacifier may be deposited on the bottom face of such a cooktop or the material from which it is constituted may be strongly colored. If colored, some minimum level of transmission must nevertheless be conserved so that displays can be seen, e.g. as a result of light emitted by light-emitting diodes (LEDs) placed under the plate.

Lithium is one of the main ingredients of glass-ceramics (of the lithium aluminosilicate (LAS) type containing a solid solution of β-quartz as the main crystalline phase). At present, lithium is present in the composition of said glass-ceramics, generally at contents lying in the range 2.5% to 4.5%, and more generally contents in the range 3.6% to 4.0% by weight (expressed in terms of Li₂O), essentially as an ingredient of the solid solution of β-quartz and as a flux for the glass. At present, the supply of lithium is less reliable than it used to be. In any event, this element is becoming more expensive. The reason for this recent pressure on the availability and the price of lithium lies in the increasing demand for lithium for producing lithium batteries.

The inventors have thus sought glass-ceramic compositions that show high performance with a low lithium content. As a result of their research, they have found compositions with lithium contents that are considerably reduced compared with those of existing glass-ceramics (see below).

The prior art already describes glasses and glass-ceramics having compositions with various low contents of lithium. Thus:

-   -   from aluminosilicate glasses that do not contain lithium but         rather a high content of zinc, it is known that it is possible         to obtain glass-ceramics containing a β-quartz solid solution as         the main crystalline phase. Nevertheless, such glass-ceramics         are not transparent (they are opaque), their precursor glasses         have low viscosity at the liquidus temperature, and the heat         treatments required for crystallizing (ceramming) said precursor         glasses in order to obtain said glass-ceramics are lengthy (see         the book “Glass-ceramic technology”, 2^(nd) edition, by W.         Hölland and G. H. Beall, pp. 116-117 (Wiley 2012));     -   patent application US 2016/0174301 describes glasses having low         CTE values (CTE_(20-300° C.)<30·10⁻⁷K⁻¹), that can be suitable         material for making induction cooktops. Said glasses do not         contain alkalis in their composition. Consequently, they are         rather difficult to melt: firstly, they have high viscosities at         high temperature, and secondly they have high electrical         resistivities, so that very high voltages are needed to work         them in an electrically heated oven. Such glasses may be colored         by oxides of transition element, but the presence of such         coloring agents in those glasses can hinder melting them, by         absorbing infrared radiation;     -   patent application WO 2005/010574 discloses optical devices         comprising microlenses. A part of the devices is made of a         crystallized glass, the disclosed composition of which is broad.         The CTE considered is CTE from −40 to 80° C. The teaching of         said prior art document lies in a context far from the one of         the present application;     -   patent application WO 2015/166183 (corresponding to patent         application FR 3 020 359) describes partially crystallized glass         plates that are optionally transparent and preferably not         colored having CTE_(20-300° C.) values lying in the range         20·10⁻⁷K⁻¹ to 40·10⁻⁷K⁻¹. That document does not contain data         showing that it is possible to obtain materials having both the         indicated compositions and CTE_(20-300° C.) values that are         lower than 20·10⁻⁷K⁻¹; neither does that document contain any         data about high-temperature viscosity. The compositions         disclosed are very broad; they may contain 1% to 2%,         advantageously 1.2% to 1.8%, preferably at most 1.5% by weight         of Li₂O;     -   U.S. Pat. No. 9,446,982 describes colored transparent         glass-ceramics containing a solid solution of β-quartz as the         main crystalline phase and having lithium contents (expressed as         Li₂O) in the range 2% to less than 3% by weight (at least 2% by         weight, with reference to controlling crystallization). For the         glass-ceramics described, and with reference to the technical         problem of making said glass-ceramics compatible with them being         decorated, it is desired to obtain         CTE_(ambient temperature—700° C.) values in the range 10·10⁻⁷K⁻¹         to 25·10⁻⁷K⁻¹;     -   patent application US 2013/0085058 addresses fining glasses that         are precursors of lithium aluminosilicate (LAS) type         glass-ceramics, and more specifically avoiding reboiling within         such glasses (the only properties specified in the examples         relate to suitability for fining). Said glasses do not have more         than 10 parts per million (ppm) of sulfur (S) in their         composition. Their composition, which is free of As₂O₃ and of         Sb₂O₃, may have 1% to 6% of Li₂O. It does not contain coloring         elements. The compositions exemplified do not have ZnO, and for         the most part they have high contents of Li₂O (3.5% and 4% by         weight);     -   patent application EP 1 170 262 describes transparent         glass-ceramics of the lithium aluminosilicate (LAS) type         suitable for use as an optical waveguide element. The         compositions specified are broad; most of the example         compositions have high contents of Li₂O and of Al₂O₃, together         with low contents of SiO₂; and     -   U.S. Pat. No. 9,018,113 describes colored transparent         glass-ceramics usable as cooktops associated with induction         heating. Their composition has 1.5% to 4.2% of Li₂O; the         compositions exemplified have high contents of Li₂O (>2.9% by         weight). No data is given about the high-temperature viscosity         of the precursor glasses.

In such a context, the inventors have investigated the potential existence of transparent glass-ceramics, the composition of which has a low lithium content (less than 2% by weight of Li₂O (see below)) and that are entirely suitable for use as the material for making cooktops in a context of induction heating, and more particularly in a context of induction heating using infrared sensors for controlling heating (it is mentioned above that the maximum temperature reached by a cooktop in operation is about 400° C. (for induction heating in general) and does not exceed 300° C. (for induction heating with infrared sensors)). Such glass-ceramics need to satisfy the following specifications:

-   -   being transparent (even if they are usually highly colored): at         the intended utilization thickness (plates typically 1         millimeter (mm) to 8 mm thick, more generally 2 mm to 5 mm         thick, and often 4 mm thick), said glass-ceramics need to have         integrated transmission, TL (%) equal to or greater than 1% and         a diffusion percentage less than 2%. Transmission measurements         may be performed using a spectrometer having an integrating         sphere, by way of example. On the basis of these measurements,         the integrated transmission (TL (%)) in the visible range         (between 380 and 780 nm) and the diffusion percentage (Diffusion         (%)) are calculated using the standard ASTM D 1003-13 (under D65         illuminant with 2° observer);     -   having a CTE_(25-300° C.) lying in the range +/−25·10⁻⁷K⁻¹         (−25·10⁻⁷K⁻¹≤CTE≤+25·10⁻⁷K¹) and preferably in the range         +/−20·10⁻⁷K⁻¹ (−20·10⁻⁷K⁻¹≤CTE≤+20·10⁻⁷K⁻¹), so as to be         acceptable for use with induction heater means, and more         particularly induction heater means associated with infrared         sensors (it has to be understood that said CTE is inferior or         equal to +25·10⁻⁷K⁻¹, advantageously inferior or equal to         +20·10⁻⁷K⁻¹, in the spirit of what has been specified above         about the teaching of the prior art), and     -   having a precursor glass that possesses advantageous properties,         even the same advantageous properties as the precursor glasses         for prior art glass-ceramics containing a higher content of         Li₂O; i.e.:         -   said precursor glass must have a low liquidus temperature             (<1400° C.) and a high viscosity at the liquidus (greater             than 200 Pa·s, indeed greater than 400 Pa·s, preferably             greater than 500 Pa·s), thereby facilitating forming;             and/or, advantageously, and         -   said precursor glass must possess a low viscosity at high             temperature (T_(30 Pa·s)<1640° C.), thereby facilitating             fining.

In other respects it is highly appreciated for said precursor glass to be capable of being transformed into glass-ceramic in a short length of time (<3 hours (h)), and preferably in a very short length of time (<1 h), and/or, advantageously and, to also have an electrical resistivity, at a viscosity of 30 pascal seconds (Pa·s), of less than 50 ohm centimeters (Ω·cm) (preferably less than 20 Ω·cm). The man skilled in the art will understand (in view of the composition of the glass-ceramics below stated) that obtaining these two last properties, which are opportunely required for the precursor glass, raises no particular difficulty.

It is also particularly interesting for the transparent glass-ceramics aimed to have their composition free of As₂O₃ and Sb₂O₃ (except for inevitable traces).

The inventors have established that such glass-ceramics exist with a composition that therefore contains little lithium (less than 2% by weight of Li₂O) and that satisfies the above specifications. Said glass-ceramics constitute the first aspect of the present application. In characteristic manner, these glass-ceramics have the following composition, free of arsenic oxide and antimony oxide, except for inevitable traces, expressed in percentages by weight of oxides:

62% to 68% of SiO₂;

17% to 21% of Al₂O₃;

1% to <2% of Li₂O;

1% to 4% of MgO;

1% to 6% of ZnO;

0 to 4% of BaO;

0 to 4% of SrO;

0 to 1% of CaO;

1% to 5% of TiO₂;

0 to 2% of ZrO₂;

0 to 1% of Na₂O;

0 to 1% of K₂O;

with Na₂O+K₂O+BaO+SrO+CaO≤6%;

optionally up to 2% of at least one fining agent comprising SnO₂; and

optionally up to 2% of at least one coloring agent.

The following may be specified concerning each of the ingredients involved (or potentially involved) in the above-specified composition at the specified contents (the extreme values of each of the ranges specified (above and below) being included in said ranges).

-   -   SiO₂ (62%-68%): the content of SiO₂ (≥62%) must be suitable for         obtaining a precursor glass that is sufficiently viscous to         limit problems of devitrification. The content of SiO₂ is         limited to 68% insofar as the greater the content of SiO₂, the         greater the high-temperature viscosity of the glass, and thus         the glass is more difficult to melt.     -   Al₂O₃ (17%-21%): the presence of ZnO and of MgO at the specified         (rather large) quantities makes it critical to control the         content of Al₂O₃ in order to limit devitrification phenomena.         Excessive quantities of Al₂O₃ (>21%) make the composition more         likely to devitrify (into mullite crystals or others) (see         comparative example 15), which is not desirable. Conversely,         quantities of Al₂O₃ that are too small (<17%) are unfavorable to         nucleation and to the formation of small β-quartz crystallites.         An Al₂O₃ content in the range 17.5% to 19% (bounds included) is         advantageous.     -   Li₂O (1% to <2%): the inventors have found that it is possible         to obtain glass-ceramics satisfying the requirements of the         above specifications while limiting the content of Li₂O to less         than 2% (and thus substantially limiting said content). Said         content is advantageously at most 1.9% (≤1.9%). The minimum         quantity of 1% is nevertheless necessary in order to obtain a         material that is transparent, to keep a low high-temperature         viscosity, and to keep satisfactory devitrification         characteristics. This minimum quantity is advantageously 1.5%.         Thus, a Li₂O content in the range 1.5% to 1.9% (bounds included)         is most particularly preferred.     -   MgO (1% to 4%) and ZnO (1% to 6%): the inventors have obtained         the looked-for result by making joint use of these two elements,         in the specified quantities, as partial substitutes for Li₂O         (present from 1% to less than 2%).

MgO: this element decreases high-temperature viscosity. It forms part of the solid solution of β-quartz. It has less impact on devitrification than ZnO (see below), but it greatly increases the CTE of the glass-ceramics (see comparative example 18). That is why its content should lie in the range 1% to 4%, advantageously in the range 1% to 3%.

ZnO: this element also serves to reduce the high-temperature viscosity of the glass and also forms part of the solid solution of β-quartz. Compared with Li₂O, it increases the CTE of the glass-ceramic, but it does so only moderately, thus making it possible to obtain glass-ceramics with CTE values less than 25·10⁻⁷K⁻¹, or indeed less than 20·10⁻⁷K⁻¹. When present in too great a quantity, it gives rise to unacceptable devitrification. In preferred manner, its content lies in the range 1% to 4%, and in very preferred manner in the range 3% to 4%.

-   -   TiO₂ (1% to 5%) and ZrO₂ (0 to 2%): ZrO₂ is opportunely (but not         compulsorily) present. In reference to its efficiency, when it         is present, it has generally to be present at levels of at least         0.1%. Otherwise stated, ZrO₂ is “not present” or is efficiently         present, generally at a level in the range 0.1 to 2%. These         elements, TiO₂ and ZrO₂, enable the glass to nucleate and enable         a transparent glass-ceramic to be formed. The joint presence of         these two elements makes it possible to optimize nucleation. Too         great a content of TiO₂ makes it difficult to obtain a         transparent glass-ceramic. TiO₂ is advantageously present at a         content lying in the range 2% to 4%. Too great a content of ZrO₂         leads to unacceptable devitrification. ZrO₂ is advantageously         present at a content lying in the range 0.5% to 2%, very         advantageously it is present at a content lying in the range 1%         to 2%.     -   BaO (0 to 4%), SrO (0 to 4%), CaO (0 to 1%), Na₂O (0 to 1%), and         K₂O (0 to 1%): these elements are optionally present. To have an         effect, each of said elements, when it is present, is generally         present at levels of at least 100 ppm. Otherwise stated, BaO is         “not present” or is efficiently present, generally at a level in         the range 0.01 to 4%; SrO is “not present” or is efficiently         present, generally at a level in the range 0.01 to 4% (see         however later); CaO is “not present” or is efficiently present,         generally at a level in the range 0.01 to 1%; Na₂O is “not         present” or is efficiently present, generally at a level in the         range 0.01 to 1%; and K₂O is “not present” or is efficiently         present, generally at a level in the range 0.01 to 1%. These         elements remain in the residual glass after crystallization.         They reduce the viscosity of the glass at high temperature, they         facilitate dissolution of the ZrO₂ (when it is present) and they         limit devitrification into mullite, but they also increase the         CTE of the glass-ceramics. That is why the sum of these elements         must be equal to or less than 6%. It may be observed that SrO is         generally not present as an added raw material, given that it is         an expensive material. In such a context (SrO not present as         added raw material), if SrO is present, it is only present as         inevitable traces (<100 ppm), brought in as an impurity with at         least one of the raw materials used or in the cullet used.     -   Fining agents: the composition of the glass-ceramics         advantageously includes at least one fining agent comprising         SnO₂. When present, said at least one fining agent is present at         an effective quantity (for performing chemical fining), which         conventionally does not exceed 2% by weight. It is thus         generally present in the range 0.05% to 2% by weight.

In a particularly opportune manner, for environmental reasons, fining is obtained by using SnO₂—generally with 0.05% to 0.6% by weight of SnO₂, and more particularly with 0.15% to 0.4% by weight of SnO₂—within the composition of the glass-ceramics of the present application which contains neither As₂O₃ nor Sb₂O₃, or which contains only inevitable traces of at least one of these toxic compounds (As₂O₃+Sb₂O₃<1000 ppm). If traces of at least one of these compounds are present, they are present as contamination; by way of example, this may be due to the presence of recycled materials of the cullet type (derived from old glass-ceramics fined with these compounds) in the charge of raw materials able to vitrify. Under such circumstances, the co-presence of at least one other fining agent, such as CeO₂, a chloride, and/or a fluoride is not excluded but, preferably, SnO₂ is present as the single fining agent.

It should be observed that the absence of an effective quantity of chemical fining agent(s), or indeed the absence of any chemical fining agent, is not completely to be excluded; fining can then be performed thermally. This non-excluded variant is nevertheless not preferred in any way.

-   -   Coloring agents: the composition of the glass-ceramics         advantageously includes at least one coloring agent. Ii is above         mentioned that in the context of cooktops, it is appropriate to         mask elements that are arranged under said cooktops. Said at         least one coloring agent is present in an effective amount         (generally at at least 0.01% by weight), it is conventionally         present at levels of at most 2% by weight, or indeed at levels         of at most 1% by weight. Said at least one coloring agent is         conventionally selected from oxides of transition elements         (e.g., V₂O₅, CoO, Cr₂O₃, Fe₂O₃ (see below), NiO, . . . ) and of         rare earths (e.g., Nd₂O₃, Er₂O₃, . . . ). In preferred manner,         vanadium oxide V₂O₅ is used since said vanadium oxide leads to         low absorption in the glass, which is advantageous for melting.         The absorption, it makes possible, is generated during the         ceramming treatment (during which it is partially reduced). It         is particularly advantageous to combine V₂O₅ with other coloring         agents such as Cr₂O₃, CoO, or Fe₂O₃ (see below), since that         enables transmission to be modulated. The inventors have         observed that by reducing the Li₂O content, smaller quantities         of V₂O₅ are needed for obtaining the same coloring, which is         also advantageous from a cost point of view (since V₂O₅ is an         element that is quite expensive). With reference to the         requirements set out below (formulated for the utilization         thickness, typically in the range 1 mm to 8 mm, more generally         in the range 2 mm to 5 mm, and often 4 mm):     -   to have an integrated transmission (TL) less than 10%,         preferably less than 4%;     -   while maintaining transmission:

+ at 625 nanometers (nm) (T_(625 nm)) greater than 1%, thus making it possible to pass light, for display purposes, from an LED emitting in the red and placed under the plate (cooktop),

+ at 950 nanometers (nm) (T_(950 nm)) lying in the range 50 to 75%, thus enabling infra-red electronic touch controls to be used, which emit and receive at said wavelength,

the combination (weight % relative to the total composition) of coloring agents as set out below has been found to be particularly advantageous:

V₂O₅ 0.005% to 0.1% Fe₂O₃ 0.01% to 0.32% Cr₂O₃ 0 to 0.1% CoO 0 to 0.1%.

Among the coloring agents, Fe₂O₃ has a special place. It has an effect on color and it is actually often present, in a less or more important amount, as an impurity (e.g. coming from the raw materials). It is however not excluded to add some Fe₂O₃ to adjust the color. Its acceptable presence “in large quantity” in the composition of glass-ceramics of the present application makes it possible to use raw materials that are less pure and thus often less expensive.

The above-identified ingredients involved, or potentially involved, in the composition of glass-ceramics of the present application (SiO₂, Al₂O₃, Li₂O, MgO, ZnO, TiO₂, ZrO₂, BaO, SrO, CaO, Na₂O, K₂O, fining agent(s) (comprising SnO₂), and coloring agent(s)) can indeed represent 100% by weight of the composition of glass-ceramics of the present application, but, a priori; the presence of at least one other compound is not to be totally excluded, providing it is at low quantity (generally less than or equal to 3% by weight) and does not substantially affect the properties of the glass-ceramics. In particular, the following compounds may be present, at a total content of less than or equal to 3% by weight, each of them being present at a total content less than or equal to 2% by weight: P₂O₅, B₂O₃, Nb₂O₅, Ta₂O₅, WO₃, and MoO₃.

The above-identified ingredients involved, or potentially involved, in the composition of glass-ceramics of the present application (SiO₂, Al₂O₃, Li₂O, MgO, ZnO, TiO₂, ZrO₂, BaO, SrO, CaO, Na₂O, K₂O, fining agent(s) (comprising SnO₂), and coloring agent(s)), thus represent at least 97% by weight, or indeed at least 98% by weight, or indeed at least 99% by weight, or even 100% by weight (see above) of the composition of glass-ceramics of the present application.

The glass-ceramics of the present application thus contain SiO₂, Al₂O₃, Li₂O, ZnO, and MgO as essential ingredients for the solid solution of (β-quartz (see below). This solid solution of β-quartz represents the main crystalline phase. This solid solution of β-quartz generally represents more than 80% by weight of the total crystallized fraction. It generally represents more than 90% by weight of said total crystallized fraction. The size of the crystals is small (typically less than 70 nm), which enables the glass-ceramics to be transparent (integrated transmission ≥1% and diffusion <2%).

Glass-ceramics of the present application contain about 10% to about 40% by weight of residual glass.

In a second aspect, the present application provides articles that are constituted at least in part of a glass-ceramic of the present application as described above. Said articles are optionally constituted in full out of a glass-ceramic of the present application. Said articles advantageously consist of cooktops, which are a priori bulk colored (see above). Nevertheless, that is not the only application for which they can be used. In particular, they also may constitute the material constituting cooking utensils, microwave oven plates, oven doors, whether colored or not. It will naturally be understood that the glass-ceramics of the present application are logically used in contexts that are compatible with their CTEs. Thus, cooktops are strongly recommended for use with induction heating means, particularly with induction heating means associated with infrared sensors.

In a third aspect, the present application provides aluminosilicate glasses that are precursors for the glass-ceramics of the present application, as described above. In characteristic manner, said glasses present a composition that makes it possible to obtain said glass-ceramics. Said glasses generally present a composition corresponding to that of said glass-ceramics, but the correspondence is not necessarily complete insofar as the person skilled in the art readily understands that the heat treatments applied to such glasses for obtaining glass-ceramics are likely to have some effect on the composition of the material. The glasses of the present application are obtained in conventional manner by melting a charge of raw materials able to vitrify (raw materials making them up being present in the appropriate proportions). Nevertheless, it can be understood (and will not surprise the person skilled in the art) that the charge in question may contain cullet. Said glasses are particularly interesting in that:

-   -   they have advantageous devitrification properties, in particular         compatible with using forming methods involving rolling,         floating, and pressing. Said glasses present a low liquidus         temperature (<1400° C.), and a high viscosity at liquidus (>200         Pa·s, indeed >400 Pa·s, preferably >500 Pa·s); and/or,         advantageously, and     -   they have a low viscosity at high temperature (T_(30 Pa·s)<1640°         C.).

In other respects, it has to be noted that it is possible to obtain (from said precursor glasses) the glass-ceramics of the present application by performing ceramming (crystallization) thermal cycling of short duration (less than 3 h), preferably of very short duration (less than 1 h); and that the resistivity of said precursor glasses is low (resistivity less than 50 Ω·cm, preferably less than 20 Ω·cm, at a viscosity of 30 Pa·s).

The low liquidus temperature, the high viscosity at liquidus, and the low viscosity at high temperature (see above) are particularly important.

In its last aspect, the present application provides a method of preparing an article constituted at least in part of a glass-ceramic of the present application, as described above.

Said method is a method by analogy.

In conventional manner, said method comprises heat treatment of a charge of raw materials able to vitrify (it being understood that such a charge able to vitrify may contain cullet (see above)) under conditions that ensure melting and fining in succession, followed by shaping the fined molten precursor glass (said shaping possibly being performed by rolling, by pressing, or by floating), followed by ceramming (or crystallization) heat treatment of the shaped fined molten precursor glass. The ceramming heat treatment generally comprises two steps: a nucleation step and another step of growing crystals of the solid solution of β-quartz. Nucleation generally takes place in the temperature range 650° C. to 830° C. and crystal growth in the temperature range 850° C. to 950° C. Concerning the duration of each of these steps, mention may be made in entirely non-limiting manner of about 5 minutes (min) to 60 min for nucleation and about 5 min to 30 min for growth of crystals. The person skilled in the art knows how to optimize the temperatures and the durations of these two steps as a function of the composition of the precursor glasses, in reference more particularly to the aimed transparency.

Said method of preparing an article, constituted at least in part of a glass-ceramic of the present application thus comprises in succession:

-   -   melting a charge of raw materials able to vitrify, followed by         fining the resulting molten glass;     -   cooling the resulting fined molten glass and simultaneously         shaping it to the shape desired for the intended article; and     -   applying ceramming heat treatment to said shaped glass.

The two successive steps of obtaining a shaped fined glass (precursor of the glass-ceramic) and ceramming said shaped fined glass may be performed immediately one after the other, or they may be spaced apart in time (on a single site or on different sites).

In characteristic manner, the charge of raw materials able to vitrify has a composition that makes it possible to obtain a glass-ceramic of the present application, thus having the composition by weight as specified above (advantageously including SnO₂ as a fining agent (in the absence of As₂O₃ and Sb₂O₃), preferably as the single fining agent). The ceramming performed on the glass obtained from such a charge is entirely conventional. It is mentioned above that said ceramming may be obtained in a short length of time (<3 h), or indeed in a very short length of time (<1 h).

In the context of preparing an article, such as a cooktop, the precursor glass is cut after being shaped and prior to being subjected to the ceramming treatment (ceramming cycle). It is generally also edged, rounded shaped and decorated. Such forming and decorating steps may be performed before or after the ceramming heat treatment. By way of example, the decorating may be performed by screen-printing.

The present application is illustrated below by the following examples and comparative examples.

EXAMPLES

-   -   To produce batches of 1 kilogram (kg) of precursor glass, the         raw materials, in the proportions specified in the first portion         of the table below (proportions expressed by (weight % of)         oxides) (which table is spread over several pages), were mixed         together carefully.

The mixtures were placed for melting in crucibles made of platinum. The crucibles containing said mixtures were then placed in a oven preheated to 1550° C. They were subjected therein to a melting cycle of the following type:

-   -   temperature rise from 1550° C. to 1670° C. in 1 h;     -   temperature maintained at 1670° C. for 5 h 30.

The crucibles were then extracted from the oven and the molten glass was poured onto a preheated steel plate. It was rolled on the plate to a thickness of 6 mm. Glass plates were thus obtained. They were annealed at 650° C. for 1 h and subsequently cooled down slowly.

-   -   The properties of the resulting glasses are given in the second         portion of the table below.

Viscosities were measured using a rotational viscosimeter (Gero).

T_(30 Pa·s) (° C.) corresponds to the temperature at which the viscosity of the glass was 30 Pa·s.

The resistivity of the glass was measured at high temperature, on a thickness of 1 centimeter (cm) of molten glass, using a 4-point contact RLC bridge. The table gives the resistivity measured at the temperature at which the viscosity was 30 Pa·s.

T_(liq) (° C.) is the liquidus temperature. The liquidus is given by a range of associated temperatures and viscosities: the highest temperature corresponds to the minimum temperature at which no crystal was observed, the lowest temperature corresponds to the maximum temperature at which crystals were observed.

The devitrification characteristics were determined as follows. 0.5 cubic centimeter (cm³) samples of glass were subjected to the following heat treatment:

-   -   placing in a oven preheated to 1430° C.;     -   maintaining this temperature for 30 min;     -   lowering to the test temperature, T, at a rate of 10° C./min;     -   maintaining this temperature for 17 h; and     -   quenching the samples.

The crystals present, if any, were observed by optical microscopy.

-   -   The ceramming cycle performed was as follows:     -   rapid temperature rise up to 500° C.;     -   temperature rise from 500° C. to 650° C. at a heating rate of         23° C./min;     -   temperature rise from 650° C. to 820° C. at a heating rate of         6.7° C./min;     -   temperature rise from 820° C. to the maximum temperature Tmax         (specified in the table) at a rate of 15° C./min;     -   maintaining this temperature Tmax for 7 min (in all of the         examples except example 18 (comparative example, see below) with         the ceramming treatment Ceram 1);     -   cooling down to 850° C. at 35° C./min; and     -   cooling down to ambient temperature as a function of the inertia         of the oven.

For certain examples (examples 1, 2, 4, 18 and 20) the results are given as obtained at the end of two different ceramming treatments (Ceram 1 and Ceram 2, which differ in the value of their Tmax).

The ceramming cycle Ceram 1 of example 18 (Tmax=830° C.) does not actually correspond to the “general” ceramming cycle specified above. It was as follows:

-   -   temperature rise up to 710° C. at a heating rate of 22.5°         C./min;     -   temperature maintained at 710° C. for 60 min;     -   temperature rise from 710° C. to 830° C. at a heating rate of         24° C./min;     -   temperature maintained at 830° C. for 30 min; and     -   cooling to ambient temperature as a function of the inertia of         the oven.     -   The properties of the glass-ceramics obtained are given in the         last portion of Table 1 below.

These glass-ceramics contain a solid solution of β-quartz as the main crystalline phase (as verified by X-ray diffraction) (with the exception of that of comparative example 16). Thus, the glass-ceramics of examples 5 and 6 respectively contain 96% and 95% (wt. %) of solid solution of β-quartz phase (relative to the total crystallized fraction) and the mean sizes of their β-quartz crystals respectively were 46 nm and 43 nm. The percentage of β-quartz solid solution and the mean sizes of the crystals were determined by the Rietveld method.

The CTE (coefficients of thermal expansion (from ambient temperature (25° C.) to 300° C. (CTE_(25-300° C.)) were measured on bar-shaped glass-ceramic samples with a high-temperature dilatometer (DIL 402C, Netzsch) at a heating rate of 3° C./min.

The aspect of the samples (transparency, color) is given in the table.

For some samples, total and diffuse transmission measurements were carried out at 4 mm using a Varian spectrophotometer (model Cary 500 Scan), fitted with an integrating sphere. On the basis of these measurements, the integrated transmission (TL (%)) in the visible range (between 380 and 780 nm) and the diffusion percentage (Diffusion (%)) were calculated in application of the standard ASTM D 1003-13 (with D65 illuminant and 2° observer). Transmission values (at 625 nm (T_(625 nm)), at 950 nm (T_(950 nm))) are also specified for some samples.

-   -   Examples 1 to 14 in the table illustrate the present         application. Examples 1 to 4 are preferred because of the values         for the liquidus viscosity of the precursor glasses.

Examples 15 to 21 (of the table) are comparative examples.

In example 15, the Al₂O₃ content is too high (21.48%>21%) and the observed devitrification of the glass is unacceptable (said glass does not have the required properties).

In example 16, the Li₂O and Al₂O₃ contents are too small and the Na₂O+K₂O+BaO+CaO content is too large. Only a small quantity of crystals formed during the heat treatment and they were spinel crystals and not a solid solution of β-quartz. Consequently, the CTE after ceramming was too high.

In example 17, the Li₂O, Al₂O₃, and ZnO contents are too large, the SiO₂ content is too small. Consequently, the glass possesses devitrification characteristics that are unacceptable.

In example 18, the MgO content is too large, and consequently the CTE of the glass-ceramics is too high.

In example 19, the MgO content is too small and the ZnO content is large. Consequently, the liquidus temperature is very high and the viscosity at the liquidus is too low (the glass does not have the required properties).

In example 20, the ZnO content is too small and the MgO content is high. Consequently, the CTE of the glass-ceramic is too high or the glass-ceramic shows optical properties that are unacceptable.

In example 21, the ZnO content is too high. Consequently, the high-temperature viscosity of the glass is very low and the liquidus temperature is high, so the viscosity at the liquidus is too small (the glass does not have the required properties).

TABLE Examples (wt %) 1 2 3 4 5 SiO₂ 66.71 66.61 66.51 65.97 64.10 Al₂O₃ 18.10 18.10 18.10 18.89 19.72 Li₂O 1.63 1.63 1.63 1.62 1.86 MgO 2.17 2.17 2.17 2.16 2.47 ZnO 3.08 3.08 3.08 3.07 3.56 BaO 2.47 2.47 2.47 2.46 2.46 CaO 0.44 0.44 0.44 0.44 0.44 TiO₂ 2.99 2.80 2.62 2.98 2.98 ZrO₂ 1.33 1.62 1.90 1.33 1.33 Na₂O 0.61 0.61 0.61 0.61 0.61 K₂O SnO₂ 0.30 0.30 0.30 0.30 0.30 Fe₂O₃ 0.12 0.12 0.12 0.12 0.12 V₂O₅ 0.03 0.03 0.03 0.03 0.03 Cr₂O₃ 0.02 0.02 0.02 0.02 0.02 CoO Na₂O + K₂O + BaO + CaO + SrO 3.53 3.53 3.53 3.51 3.51 T_(30 Pa · s) (° C.) 1636 1621 1619 1628 1571 T_(liq) (° C.) 1350-1366 1338-1350 1350-1366 1350-1360 1350-1372 Viscosity at T_(liq) 600-800 700-850 500-650 600-700 300-450 (Pa · s) Crystalline phase spinel zircon + spinel zircon spinel spinel that devitrifies at the liquidus temperature Resistivity at 8.4 9.4 9.9 8.8 7.9 30 Pa · s (Ω · cm) Ceram 1 Tmax (° C.) 890 900 890 880 880 Aspect transparent transparent transparent transparent transparent colored colored colored colored colored CTE_(25-300° C.) 18.4 17.6 19.7 20 17.5 (×10⁻⁷ K⁻¹) Ceram 2 Tmax (° C.) 920 920 920 Aspect transparent transparent transparent colored colored colored CTE_(25-300° C.) 17.5 16.3 18.3 (×10⁻⁷ K⁻¹) TL (%) 1 3 Diffusion (%) 1.5 1 T_(625 nm) (%) 3.1 8.3 T_(950 nm) (%) 58 64 Examples (wt %) 6 7 8 9 SiO₂ 63.70 65.34 65.65 65.45 Al₂O₃ 19.60 19.67 19.79 17.99 Li₂O 1.84 1.62 1.63 1.62 MgO 1.85 2.15 2.78 2.15 ZnO 4.77 3.06 1.84 3.06 BaO 2.45 2.46 2.47 3.48 CaO 0.44 0.44 0.44 0.63 TiO₂ 2.96 2.97 2.99 2.97 ZrO₂ 1.32 1.32 1.33 1.32 Na₂O 0.61 0.61 0.61 0.86 K₂O SnO₂ 0.29 0.30 0.30 0.30 Fe₂O₃ 0.12 0.01 0.12 0.12 V₂O₅ 0.03 0.03 0.03 0.03 Cr₂O₃ 0.02 0.02 0.02 0.02 CoO Na₂O + K₂O + BaO + CaO + SrO 3.50 3.51 3.53 4.97 T_(30 Pa · s) (° C.) 1584 1621 1604 1632 T_(liq) (° C.) 1370-1387 1350-1373 1350-1367 Viscosity at T_(liq) 250-350 500-700 500-600 (Pa · s) Crystalline phase spinel mullite + spinel mullite that devitrifies at the liquidus temperature Resistivity at 8.1 7.8 9.9 30 Pa · s (Ω · cm) Ceram 1 Tmax (° C.) 880 880 890 920 Aspect transparent transparent transparent transparent colored colored colored colored CTE_(25-300° C.) 15.8 21.3 22.4 20.2 (×10⁻⁷ K⁻¹) Ceram 2 Tmax (° C.) Aspect CTE_(25-300° C.) (×10⁻⁷ K⁻¹) TL (%) Diffusion (%) Examples (wt %) 10 11 12 13 14 SiO₂ 66.14 67.57 67.85 63.86 63.86 Al₂O₃ 18.10 18.98 18.87 19.00 19.00 Li₂O 1.63 1.28 1.84 1.84 1.84 MgO 2.17 2.49 1.75 1.75 1.75 ZnO 3.08 4.94 4.95 4.95 4.95 BaO 2.47 0.00 0.00 2.50 2.50 CaO 0.44 0.00 0.00 0.44 0.44 TiO₂ 2.99 2.62 2.63 3.02 2.62 ZrO₂ 1.90 1.75 1.75 1.35 1.75 Na₂O 0.61 0.00 0.00 0.62 0.62 K₂O 0.25 0.25 SnO₂ 0.30 0.30 0.30 0.28 0.28 Fe₂O₃ 0.12 0.03 0.03 0.09 0.09 V₂O₅ 0.03 0.04 0.03 0.03 0.03 Cr₂O₃ 0.02 0.00 0.00 0.00 0.00 CoO 0.02 0.02 Na₂O + K₂O + BaO + SrO + CaO 3.53 0.00 0.00 3.81 3.81 T_(30 Pa · s) (° C.) 1635 1610 1617 1581 T_(liq) (° C.) 1350-1366 1350-1375 1350-1375 1328-1353 1325-1355 Viscosity at T_(liq) 600-750 450-650 450-650 450-700 (Pa · s) Crystalline phase zircon + spinel mullite + spinel zircon + spinel that devitrifies at the liquidus temperature Resistivity at 8.3 12 7.8 30 Pa · s (Ω · cm) Ceram 1 Tmax (° C.) 890 975 975 880 855 Aspect transparent transparent transparent transparent transparent colored colored colored colored colored CTE_(25-300° C.) 24.7 18.1 7.8 13 12.9 (×10⁻⁷ K⁻¹) Ceram 2 Tmax (° C.) Aspect CTE_(25-300° C.) (×10⁻⁷ K⁻¹) TL (%) Diffusion (%) Comparative examples (wt %) 15 16 17 18 SiO₂ 63.55 65.81 54.21 63.03 Al₂O₃ 21.48 14.57 25.50 20.00 Li₂O 1.60 0.49 2.70 1.84 MgO 2.13 1.33 1.00 4.95 ZnO 3.04 4.70 7.70 1.75 BaO 2.44 6.24 1.00 2.50 CaO 0.44 0.99 1.30 0.45 TiO₂ 2.95 2.89 4.10 3.02 ZrO₂ 1.31 1.28 2.00 1.35 Na₂O 0.60 1.01 0.62 K₂O 0.21 SnO₂ 0.29 0.29 0.30 0.30 Fe₂O₃ 0.12 0.13 0.13 0.13 V₂O₅ 0.03 0.04 0.04 0.04 Cr₂O₃ 0.02 0.02 0.02 0.02 CoO Na₂O + K₂O + BaO + SrO + CaO 3.48 8.45 2.30 3.57 T_(30 Pa · s) (° C.) 1587 1705 1421 T_(liq) (° C.) >1400 >1370 Viscosity at T_(liq) <200 <100 (Pa · s) Crystalline phase mullite that devitrifies at the liquidus temperature Resistivity at 10.5 22.3 7.6 30 Pa · s (Ω · cm) Ceram 1 Tmax (° C.) 930 920 830 Aspect transparent transparent transparent colored colored colored CTE_(25-300° C.) 38.1 14.9 25.8 (×10⁻⁷ K⁻¹) Ceram 2 Tmax (° C.) 850 Aspect opalescent CTE_(25-300° C.) (×10⁻⁷ K⁻¹) TL (%) Diffusion (%) Comparative Examples (wt %) 19 20 21 SiO₂ 62.31 66.78 62.17 Al₂O₃ 19.93 18.13 18.33 Li₂O 1.80 1.63 1.51 MgO 0.47 2.91 1.83 ZnO 5.86 0.49 6.90 BaO 3.53 3.53 2.42 CaO 0.64 0.53 0.44 TiO₂ 2.90 3.00 3.28 ZrO₂ 1.29 1.34 1.84 Na₂O 0.59 0.95 0.60 K₂O 0.21 0.22 0.21 SnO₂ 0.29 0.30 0.29 Fe₂O₃ 0.12 0.13 0.12 V₂O₅ 0.04 0.04 0.04 Cr₂O₃ 0.02 0.02 0.02 CoO Na₂O + K₂O + BaO + SrO + CaO 4.97 5.23 3.68 T_(30 Pa · s) (° C.) 1580 1658 1561 T_(liq) (° C.) 1402-1415 1386-1402 Viscosity at T_(liq) 170-210 160-200 (Pa · s) Crystalline phase spinel spinel that devitrifies at the liquidus temperature Resistivity at 9.7 7.2 9.3 30 Pa · s (Ω · cm) Ceram 1 Tmax (° C.) 890 Aspect transparent colored CTE_(25-300° C.) 30.2 (×10⁻⁷ K⁻¹) Ceram 2 Tmax (° C.) 920 Aspect opalescent colored CTE_(25-300° C.) 24.8 (×10⁻⁷ K⁻¹) TL (%) 0.3 Diffusion (%) 8 T_(625 nm) (%) 1.2 

1. A transparent glass-ceramic containing a solid solution of β-quartz as its main crystalline phase, the composition of which, free of arsenic oxide and antimony oxide, except for inevitable traces, expressed as percentages by weight of oxides, comprises: 62% to 68% of SiO₂; 17% to 21% of Al₂O₃; 1% to <2% of Li₂O; 1% to 4% of MgO; 1% to 6% of ZnO; 0 to 4% of BaO; 0 to 4% of SrO; 0 to 1% of CaO; 1% to 5% of TiO₂; 0 to 2% of ZrO₂; 0 to 1% of Na₂O; 0 to 1% of K₂O; with Na₂O+K₂O+BaO+SrO+CaO≤6%; optionally up to 2% of at least one fining agent comprising SnO₂; and optionally up to 2% of at least one coloring agent.
 2. The glass-ceramic according to claim 1, wherein the composition comprises 1% to 1.9% of Li₂O.
 3. The glass-ceramic according to claim 1 or claim 2, wherein the composition comprises 17.5% to 19% of Al₂O₃.
 4. The glass-ceramic according to claim 1, wherein the composition comprises 1% to 3% of MgO.
 5. The glass-ceramic according to claim 1, wherein the composition comprises 1% to 4% of ZnO.
 6. The glass-ceramic according to claim 1, wherein the composition comprises ZrO₂.
 7. The glass-ceramic according to claim 1, wherein the composition comprises 0.05% to 0.6% of SnO₂.
 8. The glass-ceramic according to claim 1, wherein the composition comprises V₂O₅ as coloring agent, alone or mixed with at least one other coloring agent selected from CoO, Cr₂O₃, and Fe₂O₃.
 9. The glass-ceramic according to claim 1, having a coefficient of thermal expansion CTE_(25° C.-300° C.) in the range +/−25·10⁻⁷K⁻¹.
 10. An article constituted, at least in part, of a glass-ceramic according to claim
 1. 11. An aluminosilicate glass, precursor for a glass-ceramic, the composition of which makes it possible to obtain a glass-ceramic according to claim
 1. 12. The glass according to claim 11, having a liquidus temperature lower than 1400° C. and a viscosity at the liquidus of more than 200 Pa·s and having a viscosity of 30 Pa·s at less than 1640° C. (T_(30 Pa·s)<1640° C.).
 13. A method of preparing an article constituted, at least in part, of a glass-ceramic containing a solid solution of β-quartz as its main crystalline phase, comprising in succession: melting a charge of raw materials able to vitrify, followed by fining the resulting molten glass; cooling the resulting fined molten glass and simultaneously shaping it to the shape desired for the intended article; and applying ceramming heat treatment to said shaped glass; wherein said charge has a composition that makes it possible to obtain the transparent glass-ceramic, the composition of which, free of arsenic oxide and antimony oxide, except for inevitable traces, expressed as percentages by weight of oxides, comprises: 62% to 68% of SiO₂; 17% to 21% of Al₂O₃; 1% to <2% of Li₂O; 1% to 4% of MgO; 1% to 6% of ZnO; 0 to 4% of BaO; 0 to 4% of SrO; 0 to 1% of CaO; 1% to 5% of TiO₂; 0 to 2% of ZrO₂; 0 to 1% of Na₂O; 0 to 1% of K₂O; with Na₂O+K₂O+BaO+SrO+CaO≤6%; 0 to 2% of at least one fining agent comprising SnO₂; and 0 to 2% of at least one coloring agent.
 14. The method according to claim 13, wherein said charge of raw materials able to vitrify, free of As₂O₃ and Sb₂O₃, except for inevitable traces, contains SnO₂ as fining agent.
 15. The method of claim 14, wherein the charge of raw materials able to vitrify, free of As₂O₃ and Sb₂O₃, except for inevitable traces, comprises 0.05% to 0.6% of SnO₂.
 16. The glass-ceramic according to claim 2, wherein the composition comprises 1.5% to 1.9% of Li₂O.
 17. The glass-ceramic of claim 5, wherein the composition comprises 3% to 4% of ZnO.
 18. The glass-ceramic according to claim 6, wherein the composition 0.5% to 2% of ZrO₂.
 19. The glass-ceramic according to claim 6, wherein the composition 1% to 2% of ZrO₂.
 20. The glass-ceramic according to claim 7, wherein the composition comprises 0.15% to 0.4% of SnO₂. 